An ultrasonograph of the phase array sector scan type comprises a phased array of ultrasonic transducers that are brought into contact with an object to be examined. Ultrasonic beams are emitted into the object from the transducers at successively differing angles to make a sector scan. The resultant echo signals are processed to obtain an ultrasonogram of the object. The echo signals received by the transducers are put in phase via a delay map former having delay elements whose delay times are controlled according to the respective echo signals. Thus, the directivity pattern of reception is matched to the direction of emission of ultrasound. This delay map former consists of a combination of a matrix switch and a tapped delay circuit.
FIG. 10 is a block diagram of the conventional matrix switch that is used to build a delay map former. The switch has an address decoder 1 for decoding the incoming address signal, a control memory 2 that receives data regarding addresses from the decoder 1, and an analog switch array 3 which is controlled by the signal from the memory 2. In this specific apparatus, the array 3 consists of a matrix of 8.times.8 switches. The decoder 1 receives ADDRESS ENABLE signal AE and address signals A.sub.0 -A.sub.2. The memory 2 can consist, for example, of a memory whose contents can be rewritten, such as a random-access memory. Data items D.sub.0 -D.sub.7 are written to the memory 2 at addresses specified by the address decoder 1, according to the ADDRESS ENABLE signal. There exist 8 address lines to which the 8 data items D.sub.0-D.sub.7 are respectively written. Thus, 64 data items are stored in the memory 2. The whole contents of the memory are reset by MASTER RESET signal MR. The analog switch array 3 consists of a matrix of switches, 64 switches in this case, disposed at the intersections of vertical lines (junctors) and horizontal lines. These switches are closed or opened, according to their respective output signals from the memory 2. Indicated by V.sub.DD, V.sub.EE, and V.sub.SS are supply voltages.
Echo signals received by a plurality of ultrasonic transducers are supplied either to the vertical lines J.sub.0 -J.sub.7 or to the horizontal lines L.sub.0 -L.sub.7, respectively. Those lines which are not fed with the echo signals are connected with the taps of a tapped delay circuit (not shown). Data concerning a desired delay map is written to the control memory 2 to selectively close the switches of the array 3. Thus, the echo signals received by the transducers are delayed by periods matched to a desired directivity of reception.
In the conventional matrix switch circuit constructed as described above, the vertical and horizontal lines are selectively enabled, according to the contents of the control memory 2. Therefore, even when the contents of the memory 2 are being altered, the switches of the array are selectively closed, according to the present contents of the memory. However, the contents of the memory 2 make no meaning until the contents of the memory are fully rewritten. For this reason, it is necessary to inhibit the vertical or horizontal lines while the contents of the memory 2 are being rewritten.
In order to successively switch the directivity of reception to each of a number of characteristics in step with a sector scan using ultrasonic beams, the data stored in the memory 2 is rewritten each time one echo signal is received as shown in FIG. 11. It is now assumed that an echo signal is received from instant t.sub.1 to instant t.sub.2, using a delay map. Thereafter, neither the transmission of ultrasonic nor the reception of echo signals is performed during the interval between instant t.sub.2 and instant t.sub.3. During this interval, the data stored in the memory 2 is rewritten, according to the directivity of the next echo reception. Where a hundred 8.times.8 matrix switches are used in combination to make use of 6400 switches in total, about 40 .mu.s is taken to rewrite the contents of the memory 2. This has made it impossible to send and receive ultrasonic waves at a high rate.